GPU rendering on AMD Radeon HD 6800 series
Update:
Here are the official list of AMD card that Blender supports for OpenCL. It does not have Radeon HD 6800.

OpenCL Supported Devices

It is likely that Blender does not support HD 6800 for GPU rendering. 



I am going to assume the following:


You are using Blender 2.76 (latest as of 14/03/16).
You are using Cycles Render.


To use GPU render on Cycles Render, you need to enable OpenCL in Blender.

To do so, you need to:


Go to "User Preferences" (Ctrl+Alt+U)
Go to "System" tab.
Under "Compute Device", click on "OpenCL".
In the scroll box, choose your GPU.




When you render the next time, select GPU. 



Images are linked from:


Enabling GPU rendering for Cycles?
https://www.blender.org/forum/viewtopic.php?t=25173